This invention relates to electrophotography and more particularly to an improved method and means for making visible a reverse latent image formed by negative electrostatic charges on electrophotograhic plates or surfaces as are known in electrostatic printing. In certain forms of electrophotography, a relatively conductive backing member, such as a paper or metal sheet, has deposited upon its surface a uniform electrostatic charge. The charged surface is then exposed to a radiation pattern whereby the irradiated areas become discharged in accordance with the intensity of radiation, whereas the shielded areas remain charged, and thus form a latent electrostatic image. This latent image is then rendered visible by applying a dry powder developer or toner material suspended in an insulating carrier liquid such that the toner may be attracted and held electrostatically to the charged areas. The developed image may be fixed to the surface of the photoconductive material or transferred to another surface and fixed thereon.
In the prior art it is known to produce both positive and negative liquid developers by suspending a particulate pigment wetted with an oil or resin in an insulating carrier liquid to form the toner material to be deposited electrostatically. A positive developer contains toner material which can be attracted by negative charges and, therefore, can be used, for instance, to develop a latent image formed by such negative electrostatic charges on a charged and exposed zinc oxide coated electrophotographic paper. The resulting image is a facsimile reproduction of the original radiation pattern.
On the other hand, a negative developer contains toner material which can be attracted by positive charges or repelled by negative charges. The negative developer can be applied to a photoconductive layer containing a negative latent image to produce a reversal reproduction of the original radiation pattern by toner repulsion from the image areas holding negative charges, and by deposition of toner particles on the exposed areas which have no or little negative charge. The charged particles depositing themselves on the uncharged and lesser charged surfaces of the photoconductive layer adhere to the layer as a result of Van der Waal forces. Van der Waal forces are the attraction of the molecular forces in the photoconductive sheet or layer to the molecular structure of the toner particles. In this instance, the rate of deposition of the negatively charged toner material is inversely proportional to the magnitude of the negative surface charge retained on the photoconductive surface.
In developing reverse images, the negative toner material is sensitive enough to detect even minute residual changes in charge levels existing on the exposed photoconductive surfaces, and, consequently, such minute changes in charge level result in deposition of toner material according to the intensity of the charge on the photoconductive surface.
In prior developing methods, it has been an object to create a cut-off point in the toner which will prohibit the charged particles from adhering to areas with small charge level differences. In such methods, areas with large negative charges thereon receive no deposition of toner, and, consequently no image appears at these points. However, the present method and means for rendering visible these latent reverse images takes advantage of the inherent capability of the negatively charged particles to adhere to areas where the charge difference is only slight. In this manner, even though background staining may occur and reduce the overall contrast of the ultimately developed reproduction, images which heretofore were not visible in reproduction methods using cut-off toners are rendered visible with the present wide latitude toner. Even though a high density line image is not always obtained in marginal instances with the present toner, borderline visible images, such as produced by poor originals or microfilm reproductions, become visible to the viewer because of freedom from cut-off point in the toner.
In all types of photoreproduction of the type referred to herein, it is unavoidable that the background area on the surface of the photoconductor becomes exposed to some degree of illumination, particularly in the case of low density negatives wherein the background areas are not fully opaque and the image areas are not fully transparent. This exposure of the background areas results in a slight charge reduction. The charge level differences provide the basis for consequent toner deposition of varying degree on these areas using the wide latitude toner of the present invention. While this results in an overall loss in contrast, it, nevertheless, is desirable for the enhancement of the visual capabilities of the reproduced object in borderline areas.
Microfilm reproduction processes usually begin with a poor original which is to be projected on a photoconductive surface, resulting in small differences in illumination on a photoconductive surface. The difference may be small at either end of the spectrum; i.e., where there is either too much or too little light. The object of the present reproduction system is to take the small differences in light and reproduce them without a cut-off at all in the toner used. This permits the borderline images to be reproduced and rendered discernible to the naked eye.
The system to which the present invention is applicable repels negatively charged toner particles from highly negatively charged areas but permits them to deposit upon areas of lesser or no charge where they adhere as a result of Van de Waals forces. However, it is recognized that a poor original may produce images where strong negative charges remain in which case a conventional reversal toner, having a cut-off, would not deposit while the wide latitude toner of the present invention, having no cut-off, would deposit in sufficient quantity to produce an image which otherwise would not have been visible were the toner to have a cut-off.
Prior reversal toner compositions do not surround the charged particle with a chemically active liquid solution to enhance its ability to adhere to a slightly discharged photoconductive surface. Further, in some prior toner materials where a liquid solution is used, it does not become part of the negatively charged particle itself.
Therefore, it is a primary object of the present invention to produce an electrostatic liquid developer which contains negative toner material capable of depositing on slightly discharged, as well as uncharged, areas of a photoconductive surface.
A further object is to provide an electrostatic liquid developer for developing in the reverse sense latent images formed on a surface by negative electrostatic charges which developer contains negative toner material which has no cut-off.
A still further object is to provide an electrostatic liquid developer comprising toner material which enhances the reproduction of light or poor images upon an exposed electroconductive surface.
An additional object of the present invention is to provide an electrostatic liquid toner material which enables deposition of charged particles on uncharged and slightly discharged portions of a photostatic surface as would result from exposure to a small increment of illumination.
Still another object of the present invention is to provide an electrostatic liquid toner material wherein each charged particle mixes with a quantity of a chemically active liquid solution to enhance the ability of the particle to adhere to a slightly discharged as well as an uncharged surface.
Still another object of the present invention is to provide an electrostatic liquid toner material where a chemically active liquid solution is utilized as part of the charged particle.
A developer in accordance with the present invention consists of an insulating carrier liquid having an electrical volume resistivity in excess of 10.sup.9 ohm cm, and a dielectric constant of less than 3, toner particles suspended in said carrier liquid which particles may comprise non-resinated pigment particles or aggregates of non-resinated pigment particles containing thereon an absorbed layer of a wetting substance and a sensitizing agent. The particles are wetted by a chemically active liquid solution and a soy modified pentaerythritol alkyd resin. The pigment particles are ground in the presence of a chemically active liquid solution, causing them to separate whereupon a mixed layer of liquid solution and resin adheres to the surface of each charged particle and prevents reagglomerization. The liquid solution and resin surrounding each charged particle increases the capability of the particles to adhere to a slightly discharged or uncharged surface on a photoconductive surface.
In order that the functioning of the liquid developer formulated and used in accordance with the present invention may be fully understood, it is necessary to first describe the machanism of developing a latent image. On a surface containing latent images formed by negative electrostatic charges of varying magnitude, there is a flux of a density varying in proportion to the magnitude of negative charge. This flux can be imagined as issuing from the negative latent image areas of the surface in a density proportional to the surface charge magnitude in such area, and re-entering the surface in the exposed non-image areas containing lesser or no residual charges. The density of the re-entering flux in any area is inversely proportional to the magnitude of the residual negative charges present in the area. Positive toner particles deposit along the field line toward the negative charge and their rate of deposition is proportional to the magnitude of the negative charges or to the density of the flux issuing from the charge holding areas, whereas negative toner particles deposit along the field lines toward the point of re-entry and their rate of deposition is inversely proportional to the magnitude of the residual negative electrostatic charges existing in the exposed or partly exposed area. The particles are held to the lesser and under charged surfaces by means of molecular or Van der Waal forces.
It will thus be seen that prior art negative toner particles will not deposit on the unexposed areas retaining negative charges of a maximum magnitude, but toner deposition will take place onto all other areas where the charged magnitude has been reduced by exposure or due to other causes to a magnitude somewhat lesser than the aforesaid maximum. The rate of deposition of the negative, toner that is to say the image deposit in a given area, is, therefore, proportional to the exposure or to the percentage of reduction of the aforesaid maximum charge magnitude; i.e., to the difference between the aforesaid maximum charge magnitude in the unexposed areas, where no toner deposition takes place, and the lesser charge magnitude existing in the given area. The highest density image deposit can be thus attained in the fully exposed areas approaching 100% reduction of the aforesaid maximum charged magnitude retained in the unexposed area.
Prior art toner materials take advantage of the difference between minimum and maximum charges on a surface of photoconductive paper and deposit particles only where the magnitude of the negative charge is below a specified cut-off level. In other areas, no deposition of particles takes place and borderline images, consequently, fail to reproduce. Therefore, it is apparent that there is a need for a negative liquid developer for the production of reverse copies from negatives, which developer contains toner material which is capable of depositing onto areas where there is only a slight reduction in negative charge due to low level illumination of the image areas of the photoconductive surface as in the case of projection exposure from high density negatives.
The quality which differentiates a wide latitude reversal toner from other reversal toners is "cut-off". The present wide latitude toner has practically no cut-off, and will reproduce where there is only a very slight drop in charge on the copy paper surface.